Episode Six contains what is probably the most memorable death scene in Doctor Who history when Harrison Chase is seen off, getting ground up in his own compost machine to have his remains pumped out into the garden. Thankfully, the act of Chase being ground up like hamburger isn't seen, but his truly bloodcurdling scream is heard. In fact, the audio for the scene is one of the reasons why the sequence is so memorable. Geoffrey Burgon's score builds and builds during the preceding fight scene between The Doctor and Chase, but once Chase dies, the only sound heard for a few seconds afterward is the constant, grinding, mechanical sounds of the compost machine.
The sudden silence hammers home how ghastly and grisly Chase's fate was, but the whole scene is made as effective as it is because of the performances of Tom Baker and Elisabeth Sladen. Both are mortified by what has just occurred. The Doctor tries to explain to a bound Sarah that he "tried to save him! He was trying to pull me in!". Sarah, clearly sickened and horrified by the experience, can only barely utter "Couldn't reach..." when trying to describe her vain attempts to hit the stop button on the compost machine. Powerful, powerful stuff.
The Seeds of Doom is probably as edgy as Doctor Who ever was without treading into the territory of gratuitous violence. The acting is superb, the direction is superb, the writing is superb, the general mood and feel is superb. It's also one of those rare stories that benefits, obviously, from The Doctor's presence, but is in no way affected by his role in the story. The Krynoid pod is found in the Antarctic without The Doctor's help; Scorby and Keeler steal the remaining pod, despite The Doctor's best intentions to stop them; Keeler is infected by the Krynoid, it grows to full size, and it, and Chase's mansion, are destroyed by a UNIT air strike at the end.
Looking ahead a bit, The Seeds of Doom could be compared to another classic story, The Caves of Androzani, in that Peter Davison's swansong in Doctor Who also saw the events of the story relatively oblivious to The Doctor's presence. And, like the esteemed praise that Caves often receives in Doctor Who fandom, The Seeds of Doom might just be the greatest Doctor Who story ever made.