Blake's 7 - Season 4 (1981)

Scorpio is damaged in a collision with an asteroid and the crew need a stardrive from Federation scientist Dr. Plaxton so they can out-run Federation pursuit ships. Unfortunately Dr. Plaxton is working for pirates called the Space Rats.

Avon sets out to recruit Justin, one of Dayna's former tutors and former lover who is conducting genetic engineering experiments. However, Servalan captures Dayna and brainwashes her into hating Justin and forces Justin to share with her his discoveries.

The rebels help a top cybernetic scientist, Muller, to defect from the Federation. Whilst escaping, they discover a dead body and a mysterious box which terrifies Muller, leading to a disastrous struggle and quarantine for the Scorpio crew.

Commissioner Sleer hires a mysterious assassin to pick off the crew of the Scorpio, one by one.

The Federation sends Commissioner Sleer to Mecron II in order to investigate the various irregularities discovered in the mining operation on that planet. Belkov, the head of the mining unit, has hidden a large consignment of crystals aboard his orbiting space station. But Avon and his accomplices are also after the crystals, assisted by one of Belkov's erstwhile colleagues.

Marooned on the arid desert world of Virn, Tarrant and his enemy must confront each other, and the past, whilst trying to solve the mystery of a lost Federation expedition. Meanwhile, time is running out for the struggling Scorpio and Orac.

Keiller, an "old friend" of Avon, proposes an ingenious heist to make off with a huge Federation gold shipment. All it requires is the Scorpio, the Space Princess pleasure cruiser, an infiltration mission, and for nothing to go wrong.

The Scorpio crew are summoned by renegade Federation scientist Egrorian who wants to unleash vengeance on the Federation. He offers a powerful weapon in exchange for Orac. Avon soon suspects Egrorian is working for Servalan in another of her schemes.

The success of Avon's attempt to forge an anti-Federation alliance is threatened by the treachery of a key delegate.

Orac has pinpointed Blake's location and reveals to Avon and the crew of Scorpio that he is on the lawless planet Gauda Prime, where Blake has given up fighting against the Federation and is now a bounty hunter. However, the crew of the Scorpio suspect Blake has betrayed them and has sold out to the Federation.