Peter wakes with a start - his brief happiness was a dream. Realizing that the multi-dimensional machine is the key to getting him home, Peter turns to Walter, but the eccentric scientist refuses to help. "I lost my career and my sanity all because I tried to help another Peter," he reasons.
Olivia is on leave - at home with a migraine - and is surprised to receive a visit from Peter. He now believes that Walternate is the only one who can help him and wants to cross over into the alternate universe...
When Lincoln arrives to check on Olivia, Peter decides to use him to gain access to Walternate - he wants Lincoln to pose as his parallel world counterpart.
Meanwhile, in the alternate universe, a young boy stumbles upon a shape-shifter in a bus terminal bathroom. A cop chases the shifter - using its superhuman abilities, the creature runs up the side of a bus and makes a 30ft leap to escape, but is struck by a passing car.
Fauxlivia and Alt-Lincoln arrive on the scene, but moments later the Secretary of Defense pulls them off the case. Bewildered, Fauxlivia begins to suspect that Walternate is trying to cover up his own involvement in the shape-shifter's activities...
Back in the regular universe, Olivia, Peter and Lincoln travel to the Orpheum in Stamford, Connecticut - a soft spot between worlds. Lincoln - disguised as his alternate counterpart - uses Walter's old portal device to cross over with Peter, while Olivia waits patiently for their return...
With Peter as his 'prisoner', Lincoln requests a transfer order to Liberty Island - unable to present his identification, he claims that his 'Show-Me' card was lost. But when the real Alt-Lincoln tries to use his Show-Me, he finds that the card has been reported lost and makes enquiries.
Lincoln is quickly exposed as an impostor - he and Peter are arrested by Alt-Lincoln and Fauxlivia. Now prisoners for real, they are bundled into a vehicle en-route to the Department of Defense...
But after receiving a phone-call, their driver Agent Murphy pulls away from the convoy and shoots his partner. Pulling into a side road, Murphy prepares to shoot Peter too, but his fatally injured partner lets off one final shot. In the confusion, Lincoln tackles Murphy to the ground and shoots him dead.
Fauxlivia and Alt-Lincoln eventually track down Lincoln, but Peter has vanished. Lincoln denies killing Agent Murphy and his partner in cold blood and insists that Walternate is behind everything - he asks his captors to trace the last call made to Murphy...
Meanwhile, Peter breaks into the home of this timeline's Elizabeth Bishop - at first, she believes that a thief has broken in, but quickly recognises him as her son...
The pair briefly bond, but DoD troops arrive at the Bishop home - Elizabeth had forgotten that she tripped a silent alarm when she thought she was in danger.
Convincing Peter to surrender peacefully, Elizabeth orders the soldiers to transport them both to the DoD. Back at Fringe HQ, Fauxlivia and Alt-Lincoln break protocol, locking Lincoln in a maintenance closet while they covertly follow up his claims about Murphy.
At the DoD, Peter is warmly greeted by Walternate, who requests a few moments alone with his 'son'. Once Elizabeth has left, the Defense Secretary admits that he has been monitoring the other side and was already well aware of Peter's existence.
Peter's chief concern is returning to his own timeline, but he can't resist accusing Walternate of being the brains behind the shape-shifters. But when Walternate's assistant Brandon joins the pair, Walternate uses a special piece of tech to incapacitate him. 'Brandon' is a shape-shifter - the real Brandon was replaced weeks ago.
Turns out that Walternate isn't controlling the shifters - he suspects that many high-ranking officials in his own universe have been replaced. Walternate agrees to help Peter find his way home, but only if his 'son' returns to the other side to convinces them of his innocence. "Whoever's behind this is a threat to us all," warns Walternate.
Meanwhile, Fauxlivia has traced the call made to Murphy to an abandoned industrial park in the Flatiron District. Alt-Lincoln insists on bringing Colonel Broyles up to speed before they pursue the investigation further...
Broyles agrees to let his agents explore the industrial park, but once they depart, he makes a call, warning the true master of the shape-shifters of what's coming - it's the sinister scientist David Robert Jones, long thought dead!
Back in our universe, the Observer wakes a sleeping Olivia at the Orpheum. He's been shot and is badly wounded. "I have looked at all possible futures," he gasps. "And in every one the result is the same - you have to die."
Olivia turns away for a moment... but the Observer is gone!
• Thanks to the Baseball World Series, Fringe was delayed by a week at the tail-end of 2011. The end result - our mid-season finale was not this episode, as intended, but the solid yet somewhat underwhelming 'Wallflower'.
• Still, there's something to be said for deferred pleasure, as 'Back to Where You've Never Been' sees the show back on top form!
• Ever since 'regular' Lincoln became a permanent fixture, we've been waiting for a confrontation between that character and his alternate counterpart. It was great to see the two finally face-off this week!
• We're also loving this timeline's Elizabeth Bishop (Orla Brady) - a real bad-ass who can stare down a military squad!
• In another interesting diversion from the original timeline, this Olivia's powers are far less developed - she certainly can't cross between universes at will.
• This week's Fringe glyphs spell out the word J O N E S - signalling the resurrection of Jared Harris's once bisected villain!
• Spotting the Observer - did you catch him? Alright, so it was an easy one this week - but who shot our follically challenged chum?
• And yet another hint that Olivia is going to die - could we finally be about to discover the identity of the 'X' man that Agent Dunham mysteriously identified as her future killer, way back in last year's 'Lysergic Acid Diethylamide'?