[3] Lunar Module 32, commanded by Steve Maddox, departs from the Moon bound for Earth; its radar picks up an unidentified object shortly before re-entry.
The audience sees that this is a small alien limpet-like UFO, hiding in the Apollo 8 space wreck, which attaches to the passing LM 32 and interrupts communications with the SHADO Moonbase and modifies the spaceplane's course, causing the Lunar Module to re-enter the atmosphere at too steep an angle.
Foster survives by deliberately entering the atmosphere at a very shallow angle which prevents the limpet UFO from steepening his spacecraft's descent enough to destroy it before it detaches and returns to hiding.
Straker issues orders to destroy all space wrecks where the UFO might be located and invites Henderson to SHADO Control, having deduced that the limpet craft was some sort of lure.
[5] John Kenneth Muir regards the episode as one of the best, describing it as "fascinating" for "[realising that] the real impediment here is not the actual nemesis of Earth (the alien device), but our own human politics".