First, do no harm. Okay, maybe a little harm.

 Mersal (2017) looks like a big dumb action movie, but it's a big action movie that makes a very particular point.  It also has a surprisingly complicated plot told through a series of nested flashbacks; it's not hard to follow but it is blooming hard to explain.

After four medical professionals are kidnapped in flashy and almost magical fashion, police arrest Doctor Maaran (Vijay).  This leads to immediate public protest, because Maaran is the "5 Rupees Doctor," who will treat anyone with any ailment for only five rupees.  Naturally, he's very popular in his community.  Police inspector Rathnavel (Sathyaraj) begins his interrogation, and the movie switches to the first flashback.

Two years earlier, Maaran and his compounder/sidekick Vadivu (Vadivelu) arrive at the airport in Paris (a strange alternate Paris where everyone speaks English and nobody has a French accent) and are immediately pulled aside by security, though Maaran gets a chance to show off his medical prowess and soon everybody in the airport is applauding them.  Maaran is in Paris to accept an award for his charity work, and he takes the opportunity to deliver a speech about medical care as a human right rather than a business opportunity.  This offends Arjun (Hareesh Peradi), a wealthy and successful doctor who first tries to lure Maaran into private practice, then threatens to end his career.  Maaran gives him a well earned telling off, then leaves.

That evening, Maaran and Vadivu meet Arjun's assistant Anu (Kajal Aggarwal), and saves her from a robbery using magic tricks.  Over the next few days they grow close, and he shares his dream of succeeding as a professional magician.  He reveals that he has a stage show coming up, and invites Anu.  Since she has to stick close to Arjun, he tells her to invite him as well.  

At the show, a masked Maaran amazes the audience, then calls for a volunteer.  Arjun is selected, and Maaran kills him onstage.  The resulting investigation attracts the attention of Arjun's associate, the even more evil Doctor Daniel Arokiyaraj (S. J. Suryah.)

Cut to the more recent past, back in India.  The reclusive Maaran accepts a request for an interview from newly minted journalist Tara (Samantha Ruth Prabhu), which gives him another opportunity to publicly share his belief in the need for easily available health care.  This attracts Daniel's attention; the five rupee health care is cutting into his business, but Daniel seems to have a personal grudge against the young doctor as well, and sends thugs to take care of the problem.

Fortunately, Maaran is saved by . . . Maaran?  No, the second Maaran is Vetri (Vijay), a stage magician and vigilante who happens to be identical.  Back in the police interview, Rathnavel realizes that Vetri was the one who killed Arjun, and that it's Vetri wearing handcuffs and sitting before him.  Vetri reveals that the four kidnap victims were all responsible for the death of a young girl and the financial ruin of her parents.  She could have lived if she had reached a capable doctor ten minutes earlier, and now the kidnapped men will live if the police find them within ten minutes.  Even with Vetri's directions, they don't, but by that time Vetri has escaped.

While Rathnavel tracks down an old, blind magician in search of backstory, Maaran finds and confronts Vetri.  What;s the relationship between the two identical men?  Could they be brothers?  The answer is provided in another flashback, with Vijay playing another role, but to make a long story short, yes, they are.  Also everybody Vetri has killed so far was very bad indeed, and yes, Vadivu has been acting as a sidekick to both brothers all along.

Action movies with a social message are surprisingly common in Indian cinema, and this is a pretty good example.  Daniel is an awful person, but the true villain here is lack of widespread access to affordable and timely health care, and the movie makes absolutely sure that you know it.  Still, it never forgets to be an action movie, with some impressive set pieces.  And while the plot is very complicated and highly improbable, it does  hang together in the end.  It's an action movie, and it's big, but it's not dumb.