Windmark finds the boy's mind impossible to read and orders more invasive diagnostics to determine what Michael is.
However, Peter worries about the effects of re-dosing Olivia with potentially lethal levels of the drug, while Walter is concerned with the unknown condition of the parallel universe.
Astrid suggests using the window ("Peter") to determine the state of the parallel universe, and through this, they find that there are no signs of the Observer invasion there, giving Olivia's plan credibility.
While September returns to the lab to start assembly of the device to send Michael into the future, the Fringe team goes to an underground safehouse to prepare Olivia for crossing over.
When Olivia recovers from the injections, Walter warns her of how long she has and that jumping too fast will cause rapid withdrawal symptoms.
They arrange transport to Liberty Island and take her to the coordinates they believe Michael is being kept at; Olivia crosses over but finds too late that the boy is being taken to surgery, while she herself starts experiencing double visions of the two universes.
[3] Ramsey Isler of IGN gave "Liberty" a very positive review, calling it "an episode that showcased just how good this show can be when it's at the top of its game."