The Goa'uld Nerus warns the SGC of a plan by the Ori to establish a beachhead on the Jaffa planet Kallana from which to launch their invasion of the Milky Way. The team plans to sabotage the attack with the help of Colonel Carter, who returns from Area 51, but Vala believes they are mistaken to trust Nerus's intel.
"Beachhead" provides examples of the following tropes:
- All of the Other Reindeer: Throughout the episode, Vala is frustrated by the others constantly dismissing her, which ultimately results in her taking matters into her own hands and sacrificing herself to stop the supergate from forming.
- And Mission Control Rejoiced: Several technicians on the bridge of the Prometheus do this when Vala succeeds in destroying the Ori supergate.
- Batman Gambit: The force field was strengthened by the very weapons the Ori knew the Tauri and Jaffa would use to try to stop them.
- Big Eater: The Goa'uld Nerus, to the extent that it's practically his defining character trait.
- The Bus Came Back: Carter returns from her five-episode stint at Area 51.
- Cassandra Truth: Vala repeatedly warns the others that Nerus is bad news and more than likely lying to them, but they don't pay her much attention since they don't see him as a threat. Sure enough, it turns out Nerus is working for the Ori and deliberately feeding them false information so that the energy from their weapons ends up powering the Ori force shield.
- Continuity Nod:
- Denied Food as Punishment: Landry informs Nerus that this will be his incentive to cooperate after he's outed as a traitor.Nerus: I have helped the Ori, and when you kill me, I shall ascend.
Landry: No, you will descend to a small, dark room at the bottom of Area 51, and you stay there until you come up with a way to defeat the Ori.
Nerus: What could possibly compel me to do that?
- Enemy Mine: The team are willing to work with Nerus to prevent the Ori from gaining a foothold in the galaxy, though this turns out to be a subversion when Nerus is revealed as a mole.
- Evil Sounds Deep: Played with; Nerus stops using his "Goa'uld voice" while he's working with the team and states that they don't have to talk like that, but he briefly turns it back on once Landry realizes he's been playing them.
- Famed In-Story: Nerus is extremely excited to meet SG-1 after hearing of their accomplishments and is rather disappointed that O'Neill and Carter aren't around, though he still spends some time gushing over Teal'c and Daniel.
- Friendship Moment: Daniel tells Carter that he's glad to have her back, even if it's only temporary.
- Gadgeteer Genius: Nerus is apparently a gifted scientist and inventor, which is the only reason Landry accepts his offer of help in the first place.
- Heroes Gone Fishing: The start of the episode shows Mitchell, Daniel, Teal'c and Vala in unusually high spirits, playing a friendly earthlings-versus-aliens game of basketball on the base.
- Heroic Sacrifice: Vala pulls one by maneuvering a cargo ship into position to replace the last piece of the supergate, stopping the event horizon from forming but getting herself sucked into the Ori galaxy in the process.
- Lampshaded Double Entendre:Vala: (about the force shield) My god, it's enormous!(Daniel gives her a look)Vala: Not everything I say is innuendo.
- Les Collaborateurs: Nerus helps for the Ori to come into the Milky Way, hoping that he will be ascended.
- Made of Explodium: The Mark IX naquadria-enhanced warhead, specifically designed for the purpose of destroying stargates.
- Nice Job Breaking It, Hero!: The heroes end up accelerating the expansion of the Ori force shield by detonating the Mark IX and then repeatedly firing on it from orbit, feeding it with more energy.
- Nom de Guerre: Mitchell reveals that he went by the call sign "Shaft" back in his piloting days.
- Not So Above It All: Vala gets Teal'c to lift her into the air and bump hips with her during the basketball game.
- Not-So-Harmless Villain: With the exception of Vala, no one takes Nerus particularly seriously as he cuts a rather comical figure, but then he manages to trick them into playing right into the hands of the Ori.
- Nuke 'em: Pretty much what the team's initial plan boils down to.Mitchell: Oh, boy. And here I was trying to have a nuke-free career.
- Ominous Latin Chanting: After repelling the Jaffa attacking him, the Prior at Kallana switches from chanting in English to Latin/Alteran while raising a force field around himself and the stargate.
- Orbital Bombardment: Gerak's forces show up to bombard the force shield from space, which is exactly what the Ori were hoping for.
- Poor Communication Kills: Though she doesn't actually die, Daniel's — and to a lesser extent, the rest of the team's — repeated shutting down of Vala when she's trying to tell them about her idea prompts her to make a sacrifice that might have been avoidable otherwise.
- Put on a Bus: Vala is shipped off to the Ori galaxy.
- Putting the Band Back Together: Mitchell gleefully proclaims that he's got the band back together with Carter's return.
- Ray of Hope Ending: Things are still looking pretty bleak, and Daniel in particular is initially quite despondent at Vala's disappearance, but the team manage to thwart the Ori's first attempt at full-scale invasion and Carter thinks it's likely that Vala is still alive in the Ori galaxy.
- Shown Their Work: Surprisingly given the frequently somewhat goofy depiction of black holes in this series, this episode isn't inaccurate: if compressed sufficiently, any amount of mass will become a black hole (it's just that large stars are the only objects likely to become one naturally). Although a black hole with the mass of an Earth-sized planet would be less than 9 mm across.
- The Smurfette Principle: SG-1 remains a one-woman team, with Vala departing just as Carter makes her return, though this would be subverted in the following season.
- Suspiciously Similar Substitute: Amusingly lampshaded when Nerus is introduced to the "new" SG-1.Mitchell: Lieutenant Colonel Cameron Mitchell. Leader of SG-1.Nerus: But you're not...?Mitchell: No.Nerus: Will he be here?Mitchell: Nope.Nerus: (to Vala) And you're not even Colonel Carter, are you? This isn't at all the way I imagined it would be.
- Territorial Smurfette: Subverted; it seems like this will be the case when Carter makes an uncharacteristically snide comment about the "extra back-up singer", but she and Vala don't really have any problems for the rest of the episode and they eventually end up becoming good friends after Vala officially joins the team.
- Two of Your Earth Minutes: Mitchell warns the Prior that he has "thirty of our earth minutes" to shut down the gate before the Mark IX is detonated.Daniel: "Earth minutes"?Mitchell: Yeah, I always wanted to say that.
- Trash Talk: The team basketball game descends into this, with Daniel and Vala being the worst offenders.Vala: You boys aren't gonna let two people from different planets beat you at your own silly game, are you?Daniel: This silly game isn't over yet.Mitchell: That's my wingman!Vala: You need a new wingman.Daniel: I'm gonna kick your ass!Vala: Promises, promises.
- Unrealistic Black Hole: The Prior uses one to power the supergate. Notably, the time distortion effects that were so crucial to the plot in "A Matter of Time" don't really come up, even when the team is stood on the surface of the planet just meters away from a stargate connected to a black hole. However, to be fair, time dilation around a black hole doesn't become significant until you approach the event horizon, and the Schwarzschild radius of an Earth-sized planet is less than a centimeter.