SHIELD stands for Strategic Homeland Intervention, Enforcement, and Logistics Division -- rather a clumsy way of saying Homeland Security. Except instead of ISIS, they're fighting "inhumans" (people with paranormal powers that emerge suddenly, for various reasons).
The Big Bad is Hydra, a secret society run by reptile beings, which has attempted to intervene into world history many times, from ancient Egypt through Nazi Germany (Hitler was a Hydra stooge). Its current secret weapon is the Hive, some alien parasites merged into a single sinister being.
There's also an evil government agency that wants to kill all inhumans, regardless of whether they're on our side or not.
There are 11 agents of SHIELD, each with their own alliances, hidden agendas, and angst-ridden back stories.
1. Grand Ward (Brett Dalton, top photo), an undercover Hydra agent who is possessed by the Hive.
3. Pilot Melinda May (Wing-na Wen), who has a horribly tragic backstory.
4. Lincoln Campbell (Luke Mitchell, left), an inhuman who can manipulate electricity. He dies.
6. Leo Fitz (Iain de Caestecker, left), a weapons expert who suffers from brain damage and lacks emotion.
7. Jenna Simmons (Elizabeth Hemstring), a biochemist who was trapped on the planet Malveth for six months, and emerged psychologically damaged.
9. Bobbi Morse (Adrienne Palicki), who works undercover at Hydra.
11. Holden Radcliffe (John Hannah), a transhuman (don't ask).
David Conrad as Ian Quinn, a billionaire industrialist who tries to sell Deathlok soldiers to the U.S. military and has gravitonium (whatever that is).
Gabriel Luna as Robbie Reyes, the vigilante superhero Ghost Rider.
Matthew Willig as Lash, a forensic scientist who helps the SHIELD agents track newly-emerged inhumans.
Leave it to wishy-washy Joss Whedon to wait until the series has been established to gingerly introduce a gay character. During Season Three, Joey Guttierez (Juan Pablo Raba) was introduced, an inhuman who can melt metal (and hearts). He was adopted (and outed) by the SHIELD agents.
He even managed to go out on a date -- well, the beginning of a date, before he was called away on SHIELD business. Then, his story arc ended, his character was dropped.
I guess we'll have to be content with straight guys taking their shirts off.