In the cold open, Spike, Starlight Glimmer, and Twilight Sparkle are visiting the Crystal Empire, specifically because Twilight wants to see her niece again. When they arrive, they find the city in a state of mass hysteria due to a changeling sighting. Spike offers to join the royal guards while searching for it, but when he finds it, it's revealed that the changeling, named Thorax, has no intention of hurting anyone and simply wishes to overcome his base nature and make friends.
That is a powerful premise on its own, and it becomes even more powerful when Spike befriends Thorax and takes on the heavy task of getting the citizens of the Empire to accept him. He finds, however, that many in the Empire have deep-seated prejudices against the changelings, including Shining Armour, who remembers that changelings were the ones to nearly ruin his wedding. The idea that all changelings are so hostile is mostly ludicrous, and way the episode takes on prejudice is, at least for the most part, impressively bold. Cadance and Shining, often criticized for not being given much in the way of flaws, are shown staying largely closed-minded about Thorax, but this is never allowed to morph into flat antagonism. It's clear that they are worried about their baby being kidnapped, and this tends to weigh heavier in their mind than believing Spike.
Unfortunately, the episode's attempts to address prejudice are also where the main issue lies, as the episode is largely non-functional on an allegorical level. Thorax is consistently an exception from how changelings tend to be, making this the third time that My Little Pony has depicted non-pony races as not understanding friendship and needing to be taught to be nice by a single ponies. I figure that Thorax is meant to be a conduit for accepting those who, say, might have done wrong in the past, but this sits uneasily when Thorax is meant to contrast with every other changeling. This might be attributed to its short running time, but it's hard to apply the Crystal Ponies' prejudice against changelings to real-world prejudice. If we do try to apply it there, then Spike's concluding song about how "a changeling can change" feels uncomfortably misguided, as it doesn't allow for the possibility that those prejudices are entirely unreasonable.
But then, perhaps that's the wrong way to look at it. Thorax can't control the fact that he's born a changeling, but the episode expands upon the changelings in ways that make it easier to see why the Crystal Ponies are distrustful of changelings. Apparently, when love is nearby, changelings are overtaken by the instinct to feed off of it, which is frequently displayed by Thorax wrestling with some instinctual outburst. He's starving, and his changeling physiology keeps compelling him towards a hostile method of feeding, but his values lead him to stick with it. He never understood a way to live aside from attacking ponies in order to feed, but when he saw the mane six fighting off the changelings in Canterlot, he began to think about what it would be like to accept friendship. I'm still not entirely comfortable with how the show aggressively takes down any character who might not subscribe to the form of friendship shown by its main characters, but Thorax being inspired to seek friendship is significantly better than Moondancer having it shoved down her throat in "Amending Fences."
Even as someone who never particularly disliked Spike episodes, I have to say that he's gotten a particularly good deal this season. "Gauntlet of Fire," as unadventurous and plain as it was, depicted him in a positive and charming light, and here he's given a great deal of complexity without making him a jerk or the butt of a joke. I maintain that characters are more interesting when dealing with personal issues than when proactively displaying their best qualities, but "The Times They Are a Changeling" shows that nothing is better than doing both. Spike's status as the hero of the Crystal Empire becomes very important here, as it's that status which he needs to use to convince the Crystal Ponies to accept Thorax. And yet, it's a double-edged sword, as that limelight makes the already risky act of defending a changeling even more dangerous. For him to come through like he did required a great deal of personal strength.
In a way, Spike coming around to using his influence to help someone less fortunate than him is reminiscent of Rarity in "Spice Up Your Life," only with a much more distinctive story. However, Spike's arc here is also reminiscent of "The Cart Before the Ponies" in how Spike is constantly talked over and not believed by ponies older than him. This episode improves on that theme considerably by requiring Spike himself to stand up to the closed-minded ponies around him, as well as by giving the older ponies a more believable - if no less absurd - reason to not listen and hold the perspectives they do.
On a purely technical level, it's lovely to see the Crystal Empire again, and the episode transitions between humour and drama much more effectively than the similarly-set "Equestria Games." When it's funny, it's very funny, and when it's dramatic, it's overwhelmingly poignant, hitting like a sack of bricks with some of its most emotionally intricate scenes. The song near the end, sung by Spike, doesn't have the most inventive melody, and Spike's singing voice is far from the show's strongest. However, between the emotive lyrics, the passionate performance, and the very effective context, it's one of several emotional peaks in the episode itself, and it absolutely works. A little less in the way of expository dialogue would aid the flow, but even as is this is a very minor issue.
The theme of acceptance isn't new, but it's the details which make "The Times They Are a Changeling" stand out so much. Spike's character complexity, the expansion of changeling lore, and the stronger thematic strands make this at least one of the season's high points, if not one for the entire show. As a takedown of real-world prejudice, it doesn't entirely work, in part just because of the place changelings hold in this universe. Its take on the changelings is fascinating, but it also continues the questionable threads in how the series treats its non-pony races. However, it's a powerful, poignant, entertaining and memorable example of the show's optimism on full fire, and the same details which make the episode fail as an allegory make its ultimate message of acceptance all the more powerful.
After all, if even a changeling, conditioned by both nature and nurture to be dangerous and aggressive, can change, then we all can.