Tonight’s Doctor Who finale was very much one that rewarded loyal viewers of the programme – by which we mean you have to have been paying attention or there’s a good chance you’ll have been completely lost by certain aspects of the ending. But never fear, because we’re here to guide you through it…

The key to Bill’s salvation lies in this series’ first episode, The Pilot. The story largely took place at the university where Bill worked, and saw her meeting – and falling for – a young woman called Heather who had a desire to leave and a fascination with a strange puddle on campus. Having made a sudden exit after their first conversation, Heather promised Bill she wouldn’t leave her when they went to investigate the puddle for a second time – before being absorbed into it.

Heather spent the rest of the episode chasing Bill around time and space – to Australia, a distant planet and even the Dalek-Movellan War – before revealing herself. The Doctor explained that the puddle was a liquid spaceship in need of a pilot – and it selected Heather based on her desire to run away. But Heather’s last thought was a promise to Bill that she wouldn’t leave her – a thought she carried with her. Though Heather tried to tempt Bill with visions of the universe, the Doctor persuaded Bill to release Heather from the promise – which she did, but not before leaving one of her tears with Bill.

So, rather than being dead, Bill has now become a part of whatever alien race took Heather – though, as Heather explains, she can be given human form at any time. But with the power to freely explore the whole universe with someone who she may be in love with, will she want to? That remains the question – Heather is immensely powerful, able to travel anywhere in time and space, so theoretically Bill could have hundreds of adventures with Heather and still rejoin the Doctors at the start of the Christmas special.

But how come Bill doesn’t know about regeneration, because she clearly assumes the Doctor is dead? Well, she’s had a few brushes with it this series – the Doctor mentioned it in passing in Knock Knock, she saw him begin to regenerate in The Lie Of The Land and he brought it up when talking to her in The Eaters Of Light – but on all of those occasions, either there wasn’t time for follow-up questions or the Doctor fobbed her off. So we must assume that either he never explained it to her, or she believed him to be too far gone to regenerate, which isn’t without precedent – the third Doctor was in such a bad way that he required another Time Lord to kickstart the regeneration, while the newly born eighth Doctor was heard to remark “I was dead too long this time”, which explained the delay in regenerating from the seventh.