Season 3, in my opinion, started off a little shakily with "The Slippery Slope". Still very much watchable with a lot of great things, but did feel a bit dull and bland. Also didn't find Season 3 in general on the same level as the consistently solid Season 2 (which consisted of the series' best two episodes, "The Hostile Hospital" and "The Carnivorous Carnival", and on the whole the best of the three seasons). The season is still very much worth watching, especially for "The Penultimate Peril".
Despite as of now being lower rated than "The Slippery Slope", for me "The Grim Grotto" was actually superior. Also consider both parts equally good and definitely worth the look, and while not a 'A Series of Unfortunate Events' high-point there is more than enough to it to show what is so appealing about this series. And 'A Series of Unfortunate Events' is a series with a lot of appeal, it's not for everybody but it is from personal opinion a very enjoyable, well crafted and laudable adaptation of all thirteen books and goes worthily with the entertaining film version.
"The Grim Grotto: Part 1" looks suitably grim and eerie as ought, with one of the creepier looking settings of the series in a series full of them (like with "The Wide Window", "The Austere Academy", "The Hostile Hospital" and "The Carnivorous Carnival"). The music is both quirky and haunting, and the writing is a mix of absurdist, darkly humorous and with the right amount of tension and edge. The story, again showing respect to the source material while having enough of its own touches, generally goes at a better and more urgent pace than "The Slippery Slope" and a little more so than Part 2, though like some episodes it's a little bit of a slow starter.
On the most part the character relationships are very well done, with the animosity between Fiona and Violet being one of the more striking examples. Likewise with the suitably creepy one with Count Olaf and co, a sinister and fun lot but more so in the previous episodes. That between Klaus and Fiona in both parts felt very underdeveloped though. The moral dilemmas in both parts are very interesting and surprisingly well balanced, and really love that the Hook-Handed Man is a more fleshed out character in Season 3.
All the performances are just fine, Kassius Nelsons' spirited performance in a headstrong if not what one calls likeable (not the intention though) being a standout. Lucy Punch and Usman Ally also.
In conclusion, a lot to admire here. 8/10
