As we learn, both couples have been together for many years and appear to still be at the height of their happiness. It's charming, in fact, to see how much joy Mara and Daniel still get from each other's company, and likewise, how Lydia and Fermina reminisce with the same excitement as when they talk of the future. It's as if the rest of the world had faded away.
Then, the world crashes back to focus in the next scene when there's an outbreak of SARS, and all four --- who are epidemiologists and vaccinologists -- are called back into work to find and cure and contain the disease. Daniel's family lives where the outbreak was first discovered, igniting perhaps a greater sense of urgency in him.
As the days go on, the casualties grow, but that seems to be more of a side note in this soap opera of a play. The happy couples turn out to be -- well, not so happy. Mara wants a child and will do anything to get one, but Daniel is completely opposed and unprepared to be a father. Blair and Withers both deliver nuanced performances but the argument-based dialogue comes off as a little too slick to be the real utterances of a couple in their difficult situation.
Meanwhile, Lydia finds out a secret from Fermina's past that has her question their relationship. There's also an odd scene where Lydia attacks Mara for being Christian, and therefore in her eyes, responsible for the Pope's homophobic views. It comes almost out of nowhere and fails to make any significant point outside of what we already know.
Maguire might be trying to say something about life in theory as opposed to how life is really lived, but Instinct ultimately feels like we're being forced to sit through irrelevant conversations while ignoring what's really at stake.