In Marvel's Agents of S.H.I.E.L.D. we know a blue alien has been seen. His blood was used in a serum to help bring back Phil Coulson when he died. The serum had a strange effect on Coulson. The same serum was used on Skye (Chloe Bennet) with no strange side effects.
After Coulson was injected, we started getting strong urges to carve alien symbols all the time. In Episode 207, "The Writing on the Wall," it was discovered the symbols were actually a 3D map for the location of a certain city. Many speculated the city was Attilan, the home of the Inhumans.
In this week's episode, "The Things We Bury," we saw flashbacks to Skye's mother, who was 'special' and didn't age normally before she was killed. This explains why Skye didn't have a strange reaction to the serum. We also found out, from her crazy father (Kyle MacLachlan) that her real name isn't Skye.
As for the blue alien, the show's executive producer, Jeffrey Bell, has confirmed to TV GUIDE that the alien is in fact part of the Kree race. Those unfamiliar with the race but have seen Guardians of the Galaxy should be familiar with Ronan the Accuser, a member of the race.
"Our people don't know anything about the Kree or that there's a planet full of them," notes executive producer Jeffrey Bell. "What they do know is that the strange carvings created by Coulson after he was injected with the Kree serum are actually the map of a city, and they need to find that city before Hydra does. But where is it? Here or on another planet?"
What do the Kree have to do with the Inhumans?
There is an Inhumans movie on the way from Marvel Studios in November 2018. The Kree were responsible for the creation of the Inhuman race when they experimented on mankind long long ago. You can read more about the Kree's history in comics HERE.
Looks like if you haven't been watching Marvel's Agents of S.H.I.E.L.D., now is the time to start. The first season is available on Blu-ray and DVD. It also begins streaming on Netflix on November 20. Just remember, it's all connected.