Moll and L'ak release a Krenim chronophage (aka "time bug") on Discovery, paralyzing the ship and causing Burnham, Rayner, and Stamets to jump to random periods in Discovery's history.
Tropes:
- Arc Welding: The Krenim from the Voyager two-parter "Year of Hell" are revealed to have been a faction in the Temporal Cold War, which makes sense given their more esoteric time travel-based technology.
- Back for the Finale: Well, final season, but Airiam makes an appearance in the final jump.
- Bad Future: Rayner and Burnham land on a version of Discovery thirty years into the future, where Moll and L'ak's time bug successfully diverted the ship long enough for them to secure the Progenitor technology and sell it to the Breen, who then wiped out the Federation.
- Bavarian Fire Drill: Burnham and Rayner are sent back to Discovery while it's still in dry dock, and quickly bluff an engineer who thinks they're there for an inspection.
- Bottle Episode: Save the opening scene with its exterior shot, the entire episode takes place aboard Discovery. Zigzagged in this case, as the sets have to be redressed for each time period, and the costuming is through the roof due to the show featuring the return of the blue-pajamas uniforms from the first three seasons, not to mention those seasons' hairstyle and makeup.
- Call-Back:
- Rayner previously warned about a resurgent Breen empire at his hearing. They end up being the ones that Moll and L'ak sell the Progentior technology to in the Bad Future.
- Rayner remembers what Rhys told him last episode about his love for the 23rd-century Constitution-class.
- Captain Smooth and Sergeant Rough: The conflict between Burnham and Rayner's command styles is brought to the forefront when Burnham scolds Rayner for shutting Rhys down when he offers his insights regarding Moll and L'ak. Burnham appreciates the input from her staff; Rayner doesn't, believing that the situation requires decisive action, not debate. He comes to appreciate her style after the time jumps.
- Complete-the-Quote Title: The title is a line from the David Bowie song "Changes", which also features the line "Time may change me, but I can't trace time".
- Continuity Cavalcade: It's Discovery's greatest hits! Burnham and Rayner go through several past adventures thanks to the time bug.
- Continuity Nod: The time bug was created by the Krenim. Star Trek: Voyager established that they used weapons based on temporal science.
- Don't Touch It, You Idiot!: Rayner's first instinct is to grab the time bug when Stamets shows them where it's hidden. Fortunately for him, Stamets stops him because they need to deactivate it safely, lest a Time Crash occur. As is revealed later, his hand would have been Reduced to Dust had he actually gotten close enough to do it.
- Eiffel Tower Effect: When Burnham and Rayner arrive in the time period during the ship’s construction, the Golden Gate Bridge is visible through the void in the wall of the ready room.
- Fearful Symmetry: Despite a gap of several years, past and present Burnham have nearly identical fighting styles, at several points stalemating with the same move.
- Get Out!: Burnham, Rayner, and Stamets have to shoo everyone out of Engineering so they can work in private. At least twice, Stamets claims that there's a "spore breach" and everyone has to evacuate or they'll have mushrooms growing in their lungs (and admits to being a little disturbed that his people don't know that a "spore breach" isn't really a thing). The last time, he takes advantage of his pre-tardigrade surliness by announcing that he's really irate and needs to be left alone.
- "Groundhog Day" Loop: Played with, the time jumps are random according to the time period they arrive at but linear to Burnham and Rayner, who end each jump in the same location. So while they physically start from the same location each jump and carry knowledge they discover about the situation from prior jumps, they still have to quickly check records to see what is going on at that moment.
- It's All About Me: Moll and L'ak only care about securing their own freedom by selling the Progenitor technology to the highest bidder.
- Mental Time Travel: Thanks to his tardigrade DNA, Stamets's consciousness moves with each jump, taking over that version of himself each time, assuming there's a version there to inhabit. Unfortunately, this means he has to re-experience the injuries he suffered during the battle with Control.
- Mirror Match: Burnham has to fight her past self in the final loop, which takes place not long after she's been transferred to Discovery as a prisoner. Since they share the same fighting style, it's a close match, but present Burnham eventually neck-pinches her past self.
- Reduced to Dust: The Krenim time bug is equipped with temporal shields that create a bubble of hyper-accelerated time, which means anyone or anything that attempts to touch it will age into dust within moments. The only way to counteract it is with Technobabble involving relativity and the warp bubble of Discovery, reducing the effect; it succeeds, but Rayner's hand doesn't come out unscathed. Fortunately, 32nd century medical technology fixes him right up.
- Reset Button: Stamets determines that any changes to the timeline will only lock in once a jump occurs, so they can freely mess with things in an individual jump assuming they correct the problem before the next one. This is important in the final loop, because they have to convince their 23rd century counterparts to help in blatant violation of the Temporal Prime Directive, most notably Burnham telling Airiam when and how (vaguely) she is going to die.
- Ripple-Effect-Proof Memory:
- Because Burnham and Rayner attempted to transport at the same moment the time bug activated, they are brought back to the ready room with each time jump and retain their memories, remaining distinct from versions of themselves that exist (or don't) in those time periods.
- Stamets, meanwhile, retains his past immunity to temporal changes thanks to his tardigrade DNA. Unfortunately, he is instead subject to Mental Time Travel, which is less helpful because in several versions he is either injured, dead, or simply not present.
- San Dimas Time: The Time Bug shatters linear time for the crew but is still in progress from the time it was activated, so Burnham and Rayner have to manage things in small bites before resetting to their original positions in the ready room. Luckily, the length of time varies depending on Discovery's relative velocity during each jump, allowing a little more to be done in those cycles where the ship isn't moving. It's said to have been a weapon during the Temporal Cold War and could disable a ship for months until the device ran out of power. Adira comments at the end that six hours had passed instantly for everyone else.
- Set Right What Once Went Wrong: One of the earlier time jumps was to their relative future about 30 years on, where Discovery was a barely functioning husk outside a destroyed Federation Headquarters. Zora said the ship was out of commission for too long, allowing Moll and L'ak to get ahead of them on the Progenitor mission and the technology was sold to the Breen who quickly wiped everyone else out. Burnham and Rayner then had motivation to fix their situation faster.
- Surprisingly Realistic Outcome: The Temporal Accords may have outlawed all forms of time travel and associated technology, but illicit devices are still available on the black market. While the time bug is a known device, the rarity makes it an Outside-Context Problem until they figure out what is going on.
- Time Crash: Stamets warns that forcibly deactivating the time bug would cause every version of Discovery to exist simultaneously, which would cause all those iterations to obliterate each other. He instead has to build a device which safely neutralizes it, thereby shunting Discovery back into proper time.
- Time-Travel Tense Trouble: During the first time jump, Burnham and Rayner find themselves at the start of season 3 as Burnham in the Red Angel suit is leading Discovery through the temporal wormhole.Rayner: We've gone back in time to when you went forward to the future? That's a little confusing.
- Trust Password: In the final jump, both Burnham and Rayner have to use what they know about the 23rd century crew to convince them to let them stop the time bug.
- Burnham gets the bridge crew on her side by revealing various personal details, though it takes revealing Airiam's manner of death to seal the deal. She has far less success with herself, as while her past self might be talked into accepting that a future version of herself has traveled back in time, seeing herself with Captain's pips is a bridge too far.
- Rayner has to do the same with Rhys and past Burnham, recalling Rhys's love of the 23rd-century Constitution-class and telling past Burnham how she felt when she first stepped onto Discovery.
- Unstuck in Time: The Krenim time bug causes Discovery to randomly leap through its own relative timeline, the crew unaware of what's going on.
- Unusually Uninteresting Sight: Burnham and Rayner are wearing 32nd Century uniforms but frequently have brief interactions with characters from the 23rd Century who nonetheless don't question the foreign uniform. A random mechanic during dry dock construction accepts Raynor as a senior officer. In a time set during season 1, Linus interacts with present-day Burnham as the pariah she was at that time, but after an uncomfortable silence merely comments that red suits her.
- Written-In Absence: When the final jump takes them to 23rd century Discovery not long after Burnham was first posted there, Burnham explains that Lorca and Saru are presently on an away mission, leaving Airiam in charge.
- Xanatos Gambit: Moll and L'ak's Krenim bug. Ideally, Discovery would remain trapped jumping through its own timeline until the bug ran out of power, allowing Moll and L'ak to pursue the next clue at their leisure. In the Bad Future, by the time Starfleet figured out what happened to Discovery and rescued them, Moll and L'ak had long enough to discover every clue and sell the technology once they acquired it. Even though Discovery averts this future, they are still waylaid long enough for Moll and L'ak to have taken the lead in the race to the next clue.
- Year Outside, Hour Inside: What is relatively around an hour or so for Burnham, Rayner, and Stamets while jumping around translates to six hours in normal time once the ship is stabilized, just long enough for Moll and L'ak to get a head start on the next clue.
- You Are Better Than You Think You Are: The reason past Burnham attacks future Burnham is that she thinks there's no way she could possibly become a Starfleet Captain after disgracing herself through her mutiny at the beginning of the series, therefore the future version of her must be a shapeshifter trying to fool everyone. To get her cooperation, Rayner has to convince her through this trope.
- You Have GOT to Be Kidding Me!: Rayner’s expression when Burnham explains why Stamets is immune to the time loops.
