"Two American scientists are lost in the swirling maze of past and future ages during the first experiments on America's greatest and most secret project, the Time Tunnel. Tony Newman and Doug Phillips now tumble helplessly toward a new fantastic adventure somewhere along the infinite corridors of time."
Cassandra Truth: Time-Traveler's Tip #47: Don't tell anyone you meet in the past that you're from the future. C'mon, do you really expect them to believe you?
Nicely played with in "The Day the Sky Fell In" when Doug and Tony succumbing to truth serum lead their captors to believe they must be professional spies who have been conditioned to spout nonsense when drugged.
Changed My Jumper: Happens quite a few times, though Doug was able to avoid it in the pilot.
The Chase: In "Chase Through Time," Doug and Tony travel from 1547 to 1,000,000 A.D. to 1,000,000 B.C. in pursuit of a spy.
Cliffhanger Copout: Sometimes the context in which a cliffhanger took place would change details at the beginning of the next episode. For example, you find that the heroes weren't in as much danger as you thought they were, or, at least, that it was a different kind of danger than you thought.
Comet of Doom: The episode "End of the World" centers on the then most recent appearance of Halley's Comet in 1910 and the resulting panic. Huge, huge liberties are taken with what actually happened. In the episode, the source of the trouble is a fictional scientist who has predicted that the comet will hit the Earth, but Doug disproves this using some Hollywood Science nonsense.
Dull Surprise: Doug Phillips. Robert Colbert seems to have made a career of playing unemotional characters.
Elaborate Underground Base: The base for Project Tic-Toc (the government organization that created and operated the Time Tunnel) was hidden beneath the Arizona desert.
Eternal English: Ever person in every time period speaks perfect 20th Century English, no matter how far into the past or future the travelers go or what country they are visiting. Sometimes foreign characters will have stupid accents when the travelers end up in a place like France or Germany, but that's as far as it goes. The 2002 revival pilot solved this problem by giving its cast Universal Translators.
Failure Is the Only Option: Sure, maybe the Tic Toc team was able to transfer Doug and Tony to yet another location that they'll need to get out of sooner or later, but they never actually get them home.
Grandfather Paradox: In "The Day the Sky Fell In," Tony tries (sort of) to save his father from the attack on Pearl Harbor. Initially he sets out to only find out what really happened to him, but in another example of You Can't Fight Fate he soon tries to warn his father about the impending bombing. There is also some hand-wringing by the Project team about what happens if Tony's seven-year-old self is killed by his adult self changing history.
"Groundhog Day" Loop: In theory the entire series, since the last episode ends with Doug and Tony onboard the Titanic... exactly where they started out. The same footage is played and Senator Clark is back in the control room. However, this is an unintentional example: Martin Grams' book The Time Tunnel: A History of the Television Series states that it was done so that the last episode would lead into the summer reruns.
Identical Grandson: "Reign of Terror" features Whit Bissel playing one of Robespierre's men as well as his regular role of General Kirk. Kirk is quite confused as his family is from Scotland rather than France, but discovers that this man is from an obscure branch of it that left for mainland Europe.
Another has Carrol O'Connor as a British general in the War of 1812, and his descendant in the present day.
Identity Amnesia: In "The Death Merchant," Tony is shell-shocked and ends up fighting with the Confederates in the Battle of Gettysburg. He attacks Doug, who is fighting on the Union side. A punch (Tap on the Head) restores his memories to normal.
Obstructive Bureaucrat: Somewhat undermined in the pilot. Senator Leroy Clark is legitimately concerned that the billions they are spending is a waste of time. And his threats to shut down the project precipitate Tony making an unauthorized time transfer. But he does pitch in, and at the end he pledges his support to keep the project going until they bring Tony and Doug back.
Revival: A pilot for a reimagined Time Tunnel, complete with Tony Newman recast as a woman named Toni Newman, was made in 2002 but never broadcast; it's available as a DVD extra. The Sci Fi Channel announced another revival attempt a few years later, but it never got out of Development Hell.
The Smurfette Principle: Ann is the only female regular on the series. Kirk and Ray often disregard her suggestions and, if anyone is going to be kidnapped or held hostage, it will be Ann.
Snap Back: Tony and Doug inexplicably always end up back in the same physical state they were in when they first went into the tunnel: wardrobe changes, dirt, Clothing Damage, and in one episode even injuries disappear.
Stable Time Loop: In "The Day the Sky Fell In," Young Tony would have died if Tony hadn't gone back in time and made sure Young Tony left Pearl Harbor before the 1941 bombing.
Talking Down the Suicidal: Tony does this to Althea Hall to a certain extent in the series pilot. Althea's initial intent isn't to commit suicide; however, when the Titanic is sinking, she decides to take advantage of the opportunity because, due to a brain tumor, she's going to die soon anyway and she doesn't feel she's worthy to take a place on one of the lifeboats. Tony is luckily able to talk her out of it.
Twisted Ankle: Tony sprained his ankle in "Kill Two By Two" and has to lean on Doug for almost the entire rest of the episode, inevitably leading to him telling Doug to go ahead without him. Doug replies with an "Are you kidding?" and pulls him along anyway.