Spoiler ratings guide here (opens in a new window)

"Day 1" (or: Doctor Who?)
All the children on Earth suddenly start broadcasting a message from an alien race. That message is “we are coming”. Meanwhile, shady looking government agents – led by the man who would one day be Doctor – blow up the Hub via a surgical procedure from hell. 
Spoiler rating: 3

"Day 2" (Now With More Nudity)
Jack slowly recovers from this latest death while Gwen, Ianto and Rhys try to figure out what’s going on. Lois joins the team…basically…and the writers totally copy the plot of “Contact”. 
Spoiler rating: 2

"Day 3" (They're Heeeeeeeeerrrrrreeee)
The aliens make their demands. Torchwood eavesdrops via the worst spyware in the world and Frobisher and Johnson use Jack's newly discovered offspring as leverage. 
Spoiler rating: 3

"Day 4" (Sophie's Choice)
Jack reveals his history with the 456, the government bigwigs do a reenactment of the Wansee conference and it all ends very, very badly.
Spoiler rating: 4

"Day 5" (What's the Frequency, Kenneth?)
The 456 are finally defeated via a terrible solution that will ensure Jack will never be getting visits from estranged family members again. Meanwhile, the Americans take control of the only other, equally horrible solution and act like cocky assholes about it.
Spoiler rating: 4