"Master Chief vs the Halo" is the fifth episode of season 3.
After a short "previously" segment, the action resumes immediately after the last episode, with Ronald activating the Halo ring. However, nothing happens for a moment, until the entire ring starts shaking violently. Bill Gates attributes this to Ronald activating the ring before it reached 100% charge. Cortana mentions having seen a shutdown switch, and Oprah Winfrey reluctantly tells them where it is. As Master Chief, the Arbiter, and Cortana head to find it, 343 Guilty Spark violently attacks and kills Oprah. Ronald intercepts the heroes, who attempt to reason with him. The King reveals the Halo is not actually a doomsday device and has another purpose, one he never thought Ronald would risk. Ronald retorts by revealing he remembers the King using him to time travel to the ring, which happened 2000 years ago for him but only a few minutes ago for the King. As Ronald tells Master Chief he is important to his plan, the King suddenly shoots Ronald several times. The Arbiter finds he is not breathing, Master Chief flips the shutdown switch, and they turn to leave. After the credits, however, Ronald is revealed to be alive but badly wounded. The Halo reaches 100% charge, and he starts laughing before turning the key again, causing the Halo to emit a blast of light.
- This is the first post-midseason hiatus episode of season 3. The first post-midseason hiatus episode of season 2, "Master Chief vs the Superweapon," also involved stopping a doomsday device. In both cases, the device turned out to have a different purpose than the heroes thought.
- This is the first episode of season 3 that does not take place in two different time periods.
- This episode's "previously" segment leads directly into the action, which picks up immediately after the last scene of the previous episode and recap.
- Ronald says he and the heroes will have to have a lightsaber duel if they don't let him pass, a reference to the many lightsaber battles throughout the series.
- This title was originally going to be used for the previous episode. Halfway through production, the name was changed.
- The Halo ring key does not have a Windows logo on it when Ronald activates the ring again at the end of the episode.
- Guilty Spark going haywire and attacking Oprah when the ring is fired prematurely is a reference to the climax of Halo 3. In that game, when the Halo is fired prematurely, it begins collapsing, as in this episode.
- When the Ancients refuse to tell Master Chief where the shutdown switch is, he says this is a "god damn life or death situation," a reference to the first episode, in which he said choosing between drive-through and walking in is not a "god damn life or death situation." ("Master Chief Sucks at Ordering McDonalds")
- As with the previous "doomsday device," the Halo is revealed to have an alternate purpose that does not involve destroying the planet. ("Master Chief vs the Superweapon")