In this installment of the sci-fi series (which we assume is good as it features Daniel Mays; we've not had a chance to watch it ourselves), a whiteout of unprecedented strength threatens to destroy Forthaven. Stella and Tate enlist Tipper to help with the oncoming disaster. Fleur attempts to make peace with the ACs, and Cass leaves the settlement on a rescue mission that no one believes he can survive. As always, Liam Cunningham and Hermione Norris and it continues tomorrow. If it's Valentine-related viewing you're after, you're in luck. There's Horizon: How to Mend a Broken Heart, also on at 9.00pm BBC2. Dr Kevin Fong finds out how close scientists are to being able to repair hearts that stop working. He meets people who have undergone pioneering operations - including a man with a heart powered by a mechanical pump - and scientists who are pushing the limits on cardiac treatment. Romantic.